I don't have fish in my pond (just frogs), but I plan to buy a de-icer to help the frogs and to try to over-winter water lilies. The temperature in my area (Northern Virginia) usually doesn't go below 20 degrees. The pond is a pre-formed one that is 7 ft. long, 4 ft. wide and 18" deep. Could someone recommend a suitable wattage for the de-icer? Thanks.